The present invention relates to a process for the management of files on a non-erasable information carrier. Such carriers are in particular used for the filing of voluminous information, as a result of the low cost thereof. They tend to be increasingly used in connection with the filing of digitized documents and files.
The digital optical disk is an example of non-erasable information carrier. On such a carrier, it is possible to write an information at any time, but it is not possible to modify a written information. Such a carrier is said to be of the "WORM" type.
A file management process makes it possible to record data on an information carrier in accordance with a certain structure permitting rapid access to such data. A file management process must consequently permit the recording, reading and modification of different types of objects, including directories, data representing the content of files and descriptors for describing the characteristics of recorded files and directories.
Conventionally, the objects are recorded in the form of a tree, whose terminal nodes or knots are the files and the other nodes of the directories.
Non-erasable information carriers, such as the digital optical disk, are information carriers having a very recent development and which, in an increasing number of applications, tend to replace the conventionally used erasable information carriers, such as magnetic disks or floppy disks.
Processes for the management of files on erasable supports, such as the UNIX (registered trademark), MSDOS (registered trademark) or other processes cannot be applied to non-erasable carriers, because these processes presuppose that it is possible to rewrite any information sector. Thus, specific methods have been proposed for managing files on a non-erasable information carrier.
Certain processes presuppose the association of an erasable carrier and a non-erasable carrier. The file content informations are recorded in the non-erasable carrier and the description informations on the erasable carrier. These processes are not satisfactory because they do not make it possible to ensure the transport of the non-erasable carrier alone and do not provide an adequate security level in the case of the loss of data written on the erasable carrier.
Processes for the management of files on non-erasable carriers are also known, in which all the informations are recorded on the non-erasable carrier.
A first process, called ISDOS, is described in the ISDOS Manual 1985 of Information Storage Inc. In this process, the content informations are sequentially recorded from one of the ends of the storage space (first sector of the innermost track in the case of a digital optical disk) and the description informations are sequentially recorded from the other end of the storage space (first sector of the outermost track in the case of a digital optical disk). Thus, this process permits separation of the description informations and the content informations, so that it is possible to carry out a rapid search in the description informations and therefore rapidly obtain access to a file.
However, this process has limitations compared with processes for the management of files on an erasable carrier, in that it only permits a single directory level. Thus, the ISDOS process does not respect the tree-like structure of the directories conventionally encountered in the processes for the management of files on an erasable carrier. Thus, two different files, but which carry the same name in two different directories on an erasable carrier, are transformed by the ISDOS process into two versions of the same file. Moreover, the existence of a single directory level requires a sequential passage through all the description informations for the search for a file or a file version, so that there are excessive search times or large memory volumes.
A process for the management of files on a non-erasable carrier is also known, which permits a hierarchic recording of the directories in accordance with a tree. This process, called CDFS, is described in "A File System for Write-Once and Compact Disks", 1985, by Simson L. Garfinkel, MIT Media Lab.
In this process, the storage space allocation mechanism is very simple, because it is sequential. However, the disadvantage occurs that the directories, files and descriptors are recorded continuously and interleaved in the same space. Thus, the search for a file, which requires a sequential passage through the directories and descriptors is disturbed by the presence of very voluminous content informations. Furthermore, this process does not take into account the problem of errors on the non-erasable carrier and presupposes that even a voluminous file can be recorded in one block on the non-erasable information carrier. This hypothesis is not satisfied in practice on existing non-erasable carriers, such as digital optical disks.
Moreover, the general directory, called DIRLIST, which is the starting point for any file search method, is a voluminous structure, which is totally rewritten at the end of each transaction. This can lead to a considerable consumption of the storage space of the non-erasable carrier.
Finally, the ISDOS and CDFS processes share the defect of only taking into account conventional computerized files. They are not designed to effectively manage different data structures, such as digitized document pages of the facsimile type.
The object of the present invention is to obviate these disadvantages. The invention also aims at proposing a process for the management of files on a non-erasable carrier having a flexibility of use which is substantially identical to that of the known file management processes for erasable carriers.